PROJECT SUMMARY Coordinated regulation of cell proliferation and differentiation by signaling pathways is fundamental to the development of multicellular organisms. Failure in such regulations frequently lead to tumorigenesis or growth defects during development. The Notch signaling pathway mediated intercellular communication is critical for a wide range of biological processes during normal development and pathogenesis of numerous human diseases, including multiple types of cancer. However, its relationship with cell cycle regulation and growth control is complex since Notch can play either a tumor suppressor or an oncogenic role. Furthermore, little is known on whether and how the cell cycle machinery feeds back to influence Notch signaling. The proposed studies are based on two surprising preliminary findings on Notch regulation in the Drosophila oogenesis model system, where Notch plays a central role in controlling the switch of the cell cycle programs from mitosis to endoreplication cycles in epithelial follicle cells. We find that the Chromatin Assembly Factor 1 (CAF-1) acts as both a repressor and an activator to regulate Notch target gene expression in follicle cells. We also find that the cell cycle machinery regulates the stability of the active form of Notch, the cytoplasmic domain NICD. On the basis of these new findings, two specific aims are proposed to determine (1) the molecular mechanisms underlying CAF-1 regulation of Notch signaling and (2) the potential feedback mechanisms of the cell cycle machinery on Notch stability and activity. Both aims address important aspects of Notch regulation in relation to cell proliferation control, but can be executed separately. The outcomes of the proposed project will lead to improved understanding of how Notch signaling and cell cycle machinery interact during animal development and tissue homeostasis. The new insights into the regulation of Notch signaling in the Drosophila model system will have a positive impact on biomedical research due to the strong correlation of the Notch pathway with tumorigenesis and cancer development. The findings from these studies will provide new conceptual understandings of how signaling pathways exert their roles to ensure correct decisions in cellular and tissue growth, understandings which can be used to design novel therapies for diseases related to aberrant Notch signaling.